Inkjet printer

ABSTRACT

A printer head ( 1 ) of an ink jet printer ( 10 ), is provided with an internal stimulation system ( 31 ) with which it is possible to create in an upstream breaking position ( 11 ) of a jet ( 30 ), an upstream break-up forming in a zero potential area, drops ( 33 ) which will be used for printing, and jet sections ( 38 ) on the one hand and in a downstream breaking position ( 12 ), a break-up of the jet ( 30 ) or of sections ( 38 ) of the jet forming in a non-zero potential area, drops ( 43 ) which are recovered on the other hand.  
     A sorting system ( 35 ) common to all the jets ( 30 ) of the head provides simplification of the head and reduction of its bulkiness.

TECHNICAL FIELD

The invention is located in the field of printer heads and of continuousink jet printers. It also relates to a method for selectively projectingportions of a conducting ink jet and notably to a continuous ink jetprinting method. The method and the printer according to the presentinvention may be used in all industrial fields related to writing,notably to marking, coding, addressing, customization and decoration ofindustrial products.

STATE OF THE PRIOR ART

The typical operation of a continuous jet printer may be described asfollows. Electrically conducting ink maintained under pressure escapesfrom a calibrated nozzle. Under the action of a periodic stimulationdevice, the thereby formed ink jet is broken at regular time intervalsat a unique space location. Downstream from the position of the break ofthe jet, the continuous jet is transformed into a train of identical andregularly spaced out ink drops. A first group of electrodes, the usuallyrecognized function of which is to selectively transfer a predeterminedamount of electric charge to each drop of the jet, is placed in thevicinity of the break position.

The set of thereby selectively charged drops then crosses a secondlayout of electrodes within which a constant electric field prevails,which will change the trajectory of the charged drops.

In a first alternative so-called deviated continuous jet printer theamount of charge transferred to the drops of the jet is variabledepending on the value of an electrical potential applied to a chargingelectrode located in a formation area of the drops. The potentialapplied to the charging electrode is determined according to the writecommand. This potential matches the intended destination of the drop onthe substrate or in a recovery trough if the drop is not intended forprinting, for the drop which will pass into the electric fielddetermined by the potential of the charging electrodes. Another way forchanging the value of the electric charge allocated to each drop isdescribed in Patent Application FR 2471278 corresponding to the U.S.Pat. No. 4,346,387, consists of creating a charging electric field forexample increasing in an axial direction of the jet and of controllingthe formation point of the drops so that the potential of the breakingpoint, as in the previous case, matches the intended destination of thedrop on the substrate or in a recovery trough if the drop is notintended for printing. Each drop upon passing into the secondarrangement of electrodes with a constant field, experiences adeflection which increases with the electric charge which was previouslyallocated to it and is found directed towards a specific point of theprinting medium or towards the recovery trough. With this technology, byits multiple deflection levels, a unique nozzle may print, by segment orframe, a line of points of a given height, the entirety of a pattern.Passing from one segment to the other is performed by continuousdisplacement, perpendicularly to said segment, of the substraterelatively to the printing head.

The second alternative is that of the binary continuous jet. Thistechnique mainly differs from the previous one by the fact that thecharge level of the drops is binary. Upon passing through the deflectionelectrodes, drops are either uniformly deviated or not deviatedaccording to the charge which they have received. The printing ofcharacters or patterns therefore generally requires the use ofmulti-nozzle printing heads, the center distance of the ports coincidingwith that of the impacts on the printing medium. It should be noted thatgenerally the drops for printing are non-deflected drops, i.e., theircharge binary level is zero.

In both technologies, that of the deviated continuous jet and that ofthe binary continuous jet, the ink which is not used for marking thesubstrate is directed towards a trough or recuperator of unused ink andis recycled in an ink circuit so that it returns to the printingnozzles.

A method for breaking the jet into drops is very well described forexample in a patent bearing the number U.S. Pat. No. 4,220,958, whoseinventor is M. CROWLEY. According to the method described by CROWLEY,the jet of conducting ink passes through electrodes to which arelatively high potential is applied periodically. Under the action ofthese electrodes, the ink jet is charged. The charges are attracted bythe electrodes so that a force transverse to the jet deforms the surfaceof the jet. The axial velocity of the jet and the transverse movement ofthe surface of the jet combine in such a way that, at a certain distancefrom the electrodes, the jet is broken into a succession of drops.

In the description of the art prior to his invention, CROWLEY mentions apatent from Richard G. SWEET bearing the number U.S. Pat. No. 3,596,275.According to this citation, an important point of an ink jet printer isthe generation of drops. It is preferred that the drops be generated ata fixed frequency with constant mass and velocity. To achieve this goal,SWEET discloses three techniques which are illustrated in FIGS. 1, 2 and10 of his patent.

According to a first technique, the ink-emitting nozzles are vibrated.According to a second technique, the liquid jet iselectro-hydrodynamically excited with an electro-hydrodynamic exciter(EHD). A third technique is to impose a change in pressure on the liquidat the nozzle by means of a piezoelectric crystal introduced into acavity for feeding the nozzle. This last technique predominates in theliterature and is for example used in the IBM 6640 machine (registeredtrade mark).

In comparison with this state of the art, the invention of CROWLEYrelates to an electro-hydrodynamic exciter in which the length of theelectrodes crossed by the ink jet is equal to half the distance betweenthe drops.

Another ink jet stimulation method for transforming it into drops isdescribed, for example in U.S. Pat. No. 4,638,328 of DRAKE et al. Thisdeals with activation by thermo-resistive components.

A second so-called drop-on-demand ink projection printing family isessentially applied in office printers. This is a matter of printingtexts or graphic color patterns on paper or plastic media. Unlikecontinuous jet printing, the drop-on-demand technologies directly andexclusively generate the ink drops actually required for printing thedesired patterns. Therefore, neither electrodes nor ink recovery troughare found between the outlet face of a nozzle and the printing medium.These printers include a plurality of nozzles, each nozzle is associatedwith a stimulation device having the dual function of expelling a drop(kinetic energy) and of controlling the formation (profile of the drop).This stimulation device which is activated on demand by an electricalsignal offers two main alternatives:

“Bubble jet” technology initially developed by Canon and Hewlett-Packardis mainly applied in the field of office automation. A heated componentplaced in a conduit locally produces vaporization of the ink; the growthof the gas bubble produces the expulsion of a droplet of ink towards theprinting medium.

The “piezoelectric” technology is based on the deformation of apiezoelectric ceramic so as to generate an overpressure and thereby toproject ink drops. The fields of application of this technology concernoffice automation (Epson) or industrial printing (Trident, Xaar,Spectra).

The point density provided by these printers of the order of 600 pointsper inch results from the use of materials and manufacturing techniquesdeveloped for the micro-electronics industry.

In the field of industrial printing, the performances of continuous inkjet printing heads outclass the capacities of the drop-on-demandversions. The former provide:

a more extended usable ink range and consequently a wider variety ofprintable media,

a higher drop emission frequency and therefore an increased printingrate (about 100 kHz and a few meters per second versus about 10 kHz anda few centimeters per second),

a printing distance from the lower face of the printing head to theupper medium (about 10-30 mm versus 1 mm).

However, the simplicity of the design of the drop-on-demand printingheads is again not found in the binary continuous jet multi-nozzleprinters. The electrodes dedicated to charging the drops of each jet,may be controlled individually, by the frequency for forming the dropsand at voltage levels which may attain 350 volts. Manufacturing andjuxtaposition with a very fine step, of the whole of the nozzles andelectrodes of a printing head then cause the occurrence of majorproblems:

-   -   manufacturing and cost problems: multiplication of the high        voltage electronic circuits connected to the charging electrodes        and multiplication of these same charging electrodes induce a        complex and costly electronic control,    -   use and performance problems: the very dense high voltage        connector technology near the jet causes undesirable crosstalk,        the effect of which on the printing quality can only be limited        by reducing the rate of use of the drops, and consequently by        reducing the printing rate, and/or reducing the resolution.

In view of retaining the advantages of the binary continuous jet whilefinding a remedy to the drawbacks, one alternative consists of using asystem for charging and deviating the drops, common to all the jets.

A first invention with Vago as inventor is described in PatentApplication EP 949077 or U.S. Pat. No. 6,273,559 which provides astimulation device operating at frequency F, and driven by two voltagelevels. Depending on the voltage applied to this stimulation device, thebreaking point of the jet is produced at a point C or at a point L.Before proceeding further on, the following should be known.

Let us consider a jet subject to periodic stimulation, the latter isbroken into a train of drops with a special period called a wavelength.Inside a wavelength, several drops may be formed which accompany themain drop (the one with the largest volume). In the ink jet business,these secondary drops are called satellites. This notion differs withoutany ambiguity from the term of section which designates continuousportions of jets including at least two wavelengths. For this firstinvention of Vago, the difference in the voltage level applied to thestimulation means is such that the breaking points of the jet C and Dare separated from one another by a distance which is strictly less thanthe wavelength of the jet. The breaking point C is at a position wherethere exists a potential equal to that of the ink, so that the dropsformed at C are not charged. These uncharged drops are not subsequentlydeviated by deviation electrodes and will print the printing substrate.The breaking point L is at a position where there exists a potentialdifferent from the one of the ink, so that the drops formed at L arecharged. Theses charged drops are subsequently deviated by the deviationelectrodes and are directed towards a recovery trough in order to berecycled in the ink circuit. Point C is substantially found at halfdistance between the upstream and downstream sets of electrodes broughtto equal potentials but of opposite sign. The C-L distance is too shortfor creating sections.

Patent Application No. FR 2 799 688 with U.S. filing No. 09/685,064 asof 10.10.00, object of a second invention of Vago, the publication inthe journal Xerox Disclosure (Pincus—1982, Vol. 7, p. 23) describe acharging and sorting system based on a set of electrodes brought toconstant potentials. Fragmentation of the jet is located in the set ofelectrodes and preferentially facing a well identified electrodeaccording to whether the jet portion should be printed or collected bythe trough. During operation, the jet appears as a succession ofelectrically insulated drops, i.e., without any drop-borne electricalcharge, physically distinct, flanked by electrically charged sectionswhich are deflected towards the trough. Generation of isolated drops(with zero electrical charge) is triggered by an intermittentstimulation system not described. In a known way per se, theintermittent stimulation of a jet may be provided by anelectrohydrodynamic (U.S. Pat. No. 4,220,958—Crowley) or thermalactuator (U.S. Pat. No. 3,878,519—Eaton). In both cases, these areso-called external stimulation techniques as they consist of acting onan already formed jet. With an external stimulation technique, it ispossible to easily form an isolated drop in a jet to the extent that theliquid flows past the stimulation device, for which the radius of actionhas a small range, and two configurations appear.

In the absence of a stimulation signal, the jet is not perturbed andremains continuous up to the natural breaking position.

Application of a stimulation signal selects a perfectly defined jetportion, the length of which only depends on the forward movementvelocity of the jet and on the duration of the excitation signal. Underthe effect of the surface tension, the stimulated jet section with aproperly selected length will produce an isolated drop in the continuousjet.

In the second invention of Vago, the breaking position of the continuousjet, in order to form a drop on demand, is placed in an area where anelectrode common to all the nozzles of the printing head maintains apotential equal to that of the ink in the printing head. A chargingelectrode is placed downstream from this breaking position. As long asthe jet is not broken, because the ink used is conducting, a jet portionplaced downstream from the breaking position is found in the influencearea of the charging electrodes. On the other hand, when the drops areformed before passing through the electric field of the chargingelectrodes, they are electrically isolated and are not charged.

These uncharged drops formed on demand are not deviated by the deviationelectrodes placed downstream from the charging electrodes. They willtherefore print the printing substrate. The sections which arethemselves charged, are deviated by the deviation electrodes towards arecovery trough. In the second invention of Vago, the command forwriting a drop is not performed as in the continuous jet printers, atcharging electrodes, placed in the ink flux downstream from the nozzlesfor ejecting ink but at the stimulation means which are locateddownstream from these nozzles. Such a device in which formation of dropsin the jet is perturbed upstream from the nozzle is said to be withinternal stimulation. The first and second inventions of Vago therebyassociate the advantages of the drop-on-demand printing with those ofthe continuous jet.

DISCUSSION OF THE INVENTION

The present invention as the first and second inventions of Vago, aimsat associating the advantages of drop-on-demand printing with those ofthe continuous jet. As a reminder, these advantages notably include:

Suppression for each jet, of the set of individual electrodes forcharging the drops and of the control circuit associated with this setof individual electrodes.

Application of digital data defining the pattern to be printed, nolonger downstream from the nozzles, but upstream, at the means forstimulating the jet. These are the data which will either determine ornot the formation of the drops used for printing.

Quality of printing is thereby enhanced by suppression of the crosstalkby means of electrostatic coupling between the different jets of a sameprinting head. Further, manufacturing is simplified and the global sizeof the printing heads is reduced.

The invention also aims at these advantages but with enhancements whichwill be described hereafter.

In the device described in the second invention of Vago, the chargingelectrodes must create a charging field in a separate area from theprotection area reserved for drops intended for printing, with at themost the diameter of a drop. In this way, the shortest sections, thelength of which is about two drop diameters, before the break, have aportion located in the charging area and may be charged. Further, it ispreferable that the charging electrodes have an area of influence, thelength of which in the direction of the axis of the jet, is sufficientlylarge to ensure charging of a section proportionally to the length ofsaid section, and therefore to its mass. In this way, sections ofdifferent lengths and therefore of different masses are all deviated inan identical way and an inlet port of the recovery trough may retain areasonably small size, while ensuring recovery of all the sectionsregardless of their length.

The present invention also aims at better controlling the ink jetportions not intended for printing. It also aims at simplifying themanufacture of printing heads by loosening tolerances on the position ofthe electrodes common to all the nozzles of the head. It also aims atincreased compaction of the global dimensions of the printing head, anda larger printing distance.

According to the invention, instead of breaking up the jet, exclusivelyfor creating the drops required for printing, the jet being then dividedinto drops and jet portions, it is also broken up in a regular andcontrolled way to create drops which will for example be electricallycharged and deviated by deflection electrodes. For this, means forstimulating the jet, intended to break the jet, are capable of causingbreak-up of the jet in the two positions of the jet axially separatedfrom each other, an upstream breaking position and a downstream breakingposition, the latter being more downstream in the forward direction ofmovement of the jet than the upstream position. At the upstream breakingposition, the jet will be intermittently broken up in order to createink drops which will be used for printing. Thus, after the upstreambreaking position, the jet may be continuous from the nozzle, if nointermittent drop has been formed, or on the contrary, distributed asdrop(s) and section(s) if one or more intermittent drops have beenformed. The upstream breaking position will for example be an area inwhich electrodes maintain a potential equal to the one of the ink in theprinting head, so that the intermittent drops will not be chargedelectrically. The downstream breaking position is commented here in theexample, in an area where charging electrodes maintain a potentialdifferent from the one of the ink in the printing head so that thecontinuous drops will be charged electrically. At the downstreambreaking position, it is the jet which is broken up if there has notbeen any intermittent break-up at the upstream position, on the otherhand, if there has been a break at the upstream position, the jetsection resulting from this is continuously divided into drops. Thus,after the downstream breaking position, the jet is entirely divided intodrops. Deflection electrodes located downstream from both breakingpositions then allow sorting to be performed between the charged dropsand the uncharged drops for sending the ones to a recovery trough andthe others to a printing medium.

Thus, the invention is relative to an ink jet printer comprising:

a printing head with one or more nozzles with an accommodating head bodynotably for each nozzle,

a hydraulic path of the ink including a stimulation chamber in hydrauliccommunication with one of the printing nozzles emitting a pressurizedink jet along an axis of this nozzle,

internal means for stimulating the ink jet emitted by the nozzle,mechanically coupled with the ink accommodated in the stimulationchamber, these means acting on the jet emitted by the nozzle forbreaking up the jet in a controlled way, and

means for recovering the ink which is not received by a printingsubstrate,

a generator of electrical control signals, receiving a control signaland delivering stimulation signals to the stimulation means,

an arrangement of charging electrodes defining around the axis of thenozzle, upstream and downstream areas, the downstream area being furtheraway from the nozzle than the upstream area, upstream and downstreamelectrodes of this arrangement being connected to sources of electricpotential in order to maintain in one of the areas a potential equal tothe one of the ink found in the body of the printing head, and in theother one of these areas, a potential different from the one of the inkfound in the body of the printing head,

an arrangement of deflection electrodes axially located downstream fromthe arrangement of charging electrodes

characterized in that the generator of electrical control signalsdelivers to the stimulation means, signals intermittently causing thecontrolled break-up of the jet in a upstream breaking position locatedin the upstream area in order to intermittently form a drop, therebyseparating the jet into a drop and a section and also causing controlledbreak-up of the jet or of sections of the jet continuously in adownstream breaking position, the continuous jet emitted by the nozzlebeing thereby transformed after the downstream area in a continuoustrain of electrically charged and uncharged ink drops.

The generator of electrical control signals may be physically separatedfrom the printing head. It may also be part of it, physically. In thelatter case, the invention also relates to the printing head.

In an embodiment, the printer or the printer head according to theinvention is characterized in that the upstream electrode or thearrangement of charging electrodes is connected to the same potential asthe ink.

Thus, in this embodiment, the charged drops are the ones which resultfrom the break-up of the jet or of jet sections in the downstream area.They are deviated by the arrangement of deflection electrodes towardsmeans for recovering the ink. Each period of the periodic signal createsa mechanical reaction of the stimulation means, this reaction causingthe breaking of the jet or of jet sections in the downstream area. Eachintermittent pulse of the pulse signal creates a mechanical reaction ofthe stimulation means causing the breaking of the jet in the upstreamarea into a drop and a section. In a way known per se, the charged dropsmay be directed towards the printing substrate and the uncharged dropstowards the means for recovering the ink. In this case, it is sufficientthat the upstream breaking position, where the drops intended forprinting are formed, be in an area where an arrangement of electrodesmaintains a potential different from the one of the ink, whereas thepotential maintained in the downstream area has a value equal to that ofthe ink.

In an embodiment, the printer or the printer head according to theinvention is characterized in that the stimulation means include apiezoelectric material, the generator of electrical control signalsdelivering to the stimulation means, a continuous printing signal formedby a periodic signal with period Tb, intermittently replaced by a pulsesignal preceded and followed by transition signals.

In an embodiment, the printer or the printer head according to theinvention is characterized in that the pulse signal delivered by thegenerator of electrical control signals is formed by a pulse including 3consecutive voltage steps each connected to the next by a rising frontor a steep voltage fall.

In an embodiment, the printer or the printer head according to theinvention is characterized in that the pulse signal delivered by thegenerator of electrical control signals, is formed by a succession of 3rectangular pulses separated from each other by voltage steps with alower level than the level of the pulse with the lowest level.

In an embodiment, the printer or the printer head according to theinvention is characterized in that the periodic signal delivered by thegenerator of electrical control signals is formed by a signal, thespectrum of which consists of two lines at a first frequency and a lineat a second frequency double of the first, of other possible lines ofthe spectrum having coefficients much smaller than the coefficientsassociated with the lines of the first or second frequency, for examplea signal resulting from a combination of two sinusoidal signals. Theperiodic signal delivered by the generator of electrical control signalsmay also be formed by a combination of more than two sinusoidal signals.

In an embodiment, the printer or the printer head according to theinvention is characterized in that the sum of the durations of the pulsesignal and of the transition signals delivered by the generator ofelectrical control signals is equal to an integral number of periods ofthe periodic signal.

In an embodiment, the printer or the printer head according to theinvention is characterized in that a Helmholtz frequency of a portion ofa hydraulic path of the ink feeding a nozzle located downstream from arestrictor, has a value located outside a bandwidth of the jet issuedfrom this nozzle.

In an embodiment, the printer or the printer head according to theinvention is characterized in that the hydraulic path of the inkincludes a restrictor and in that the length of a hydraulic path betweenan inlet of the restrictor and the nozzle is less than the quarter ofthe wavelength of sound in the ink.

In an embodiment aiming at avoiding generation of undesired breaks,i.e., avoiding the formation of droplets between the drops which oneactually wants to form, and the other portions of the jet or jetsections, the printer or the printer head according to the invention ischaracterized in that the system for stimulating a jet emitted by anozzle is strictly non-resonant, i.e., the transfer function of thestimulation system is free of any resonance peaks in the bandwidth ofthe jet. As a reminder, the transfer function of the stimulation systemis defined as the relationship existing between the pressure induced bythe action of the piezoelectric component and the velocity modulationintroduced in the ejection velocity of the jet. The stimulation systemtherefore comprises not only stimulation means but also the hydraulicpath of the ink in the body of the printing head.

Explanations will be given later on, on how to obtain such a result.

In an embodiment, the printer or the printer head according to theinvention is characterized in that the stimulation means include inaddition to the piezoelectric material, a membrane which is mechanicallycoupled with it, a resonance frequency of a vibrating component formedby the membrane and the piezoelectric material, being higher than acut-off frequency of the jet.

Finally the invention also relates to a method for printing a medium bymeans of a printer according to the invention in one of its embodimentswherein an ink jet emitted by a nozzle of the printer is fractionated inorder to intermittently form first drops which impinge on the substratein order to form points, and sections,

characterized in that,

the jet or the sections resulting from the fractionation of the jet intofirst drops and sections, are fractionated into second drops, the seconddrops resulting from this last fractionation being directed towards thetrough.

SHORT DESCRIPTION OF THE DRAWINGS

Complementary explanations and an exemplary embodiment of a printer or aprinter head according to the invention, will now be given in connectionwith the appended drawings wherein,

FIG. 1 is a perspective diagram for explaining the operating mode of anink jet printer according to the invention;

FIG. 2 includes the portions a and b. Portion a is a diagram showing themethod for breaking up the jet in the situation of non-printing, portionb is a diagram showing the method for breaking up the jet in a printingsituation;

FIG. 3 includes portions a to g. Each of the portions shows a step ofthe usual method for breaking the jet;

FIG. 4 includes portions a and b. Portions a and b are graphs bearing inordinates, voltage values and in abscissae, duration values, eachshowing an example of a pulse signal, which may be applied to thestimulation means in order to obtain an intermittent break-up of thejet;

FIG. 5 includes portions a to d. Portions a to d are graphs bearing inordinates, voltage values and in abscissae, duration values, the graphin portion a is an example of a signal which may be applied to thestimulation means in order to obtain a faultless break-up of the jet inthe non-printing situation; the graph in portion c is an example of asignal which may be applied to the stimulation means in order to obtaina faultless break-up of the jet in the printing situation; the graphs ofportions b and d each illustrate a logical state of a printing controlsignal;

FIG. 6 is an example of a section of a printing head showing the part ofthe ink in a body of this head;

FIG. 7 is a graph showing the transfer function of an exemplarystimulation system. It includes in abscissae, the velocity perturbationlocally provided to the jet depending on the frequency of a mechanicalstimulation present in the ink circuit upstream from the nozzle.

DETAILED DISCUSSION OF PARTICULAR EMBODIMENTS

FIG. 1 schematically illustrates in perspective the portions of aprinter concerned by the invention. In this figure, the means fortransporting the printing medium are notably not illustrated. Thisfigure is essentially intended for explaining the operation of a printerbased on the present invention.

In the shown exemplary embodiment, the printer 10 includes one, asillustrated, or several printing heads 1. A printer head 1 including 3nozzles 29 for injecting an ink jet 30 is illustrated in FIG. 1.Actually, the number of nozzles is much larger. For each of the nozzles,a body 23 of the printing head notably includes a hydraulic path for theink and a stimulation chamber 28 which will be described in more detaillater on in connection with FIG. 6. Each stimulation chamber 28 in a wayknown per se is constantly filled with ink maintained at constantpressure by a pressurized ink supply 27. Each stimulation chamber 28includes stimulation means 31 each formed by a piezoelectric component25 and a membrane 24. A signal generator 32 for controlling stimulationmeans 31 is connected to each of the piezoelectric components 25. IMPcontrol signals intended for each of the stimulation means 31 arereceived by the circuit 32 preferably, as illustrated in FIG. 1, on aparallel bus including a route for each means 31. An ink supply circuitcommon to the chambers 28 is symbolized in this figure by arrows 14showing that ink drops 43 formed in a downstream breaking position ofthe jet 30 or of sections 38 of the jet are recovered in a trough 40common to the set of nozzles of a head and directed towardspressurization and suction means symbolized by a block 13. Such an inkcircuit feeding pressurized ink 16 to each of the inlets 27 of thechamber 28 is known per se.

Pressure exerted on the ink is sufficiently large to cause the ejectionof an ink jet 30, through each ink ejection nozzle 29, at an averagevelocity Vj. A nozzle 29 has a section, the equivalent radius of whichis equal to “a”, which is also approximately the radius of the jet 30.With the stimulation device 31, controlled by the generator ofelectrical signals 32, it is possible to create a perturbation insidethe chamber 28, causing the break-up of the jet 30 into drops 33, 43.According to the invention, the electrical stimulation signals are suchthat they intermittently cause break-up of the so-called intermittentjet in a first axial position 11 on the one hand and a second break-upof the jet in a second axial position 12 downstream from the first one,so-called continuous break-up, on the other hand. The drops 33 are thedrops resulting from the intermittent break-up and the drops 43 are thedrops resulting from the continuous break-up. Examples of signals,capable of causing the intermittent and continuous break-ups, will begiven later on. A charging electrode 35 common to all the nozzles 29 islocated downstream from the nozzles 29, in direct vicinity of the axesof the nozzles 29. In the example commented here, the charging electrode35 is formed by a stack of two electrically conducting materials 34, 37,separated by a layer 36 consisting of an electrically insulatingmaterial. The conductor 34 is the most upstream, the conductor 36 is themost downstream from the charging electrode 35. The conductor 34 isconnected to the same potential as the ink found in a chamber 28,generally the zero potential of the electrical ground. The conductor 36is connected to a non-zero electrical potential Vc, different from theone of the ink found in a chamber 28. A set 39 of deflection electrodesis found in direct vicinity of the axes of the nozzles downstream fromthe charging electrode. The set 39 of deflection electrodes is common toall the nozzles 29 of a head and is connected to a potential source sothat a uniform electric field E0 prevails, whose component perpendicularto a plane containing the axes of the nozzles 29 predominates. Arecovery trough 40 common to the set of nozzles and located downstreamfrom the set 39 of deflection electrodes and outside the axes of thenozzles 29 is used in known way for recovering the ink which is not usedfor printing. The used ink for printing is directed towards a printingmedium 41 on which each printing drop 33 forms a printing point 58.

Operation of the printing head is the following.

In the example commented here, the drops 33 are the drops which are usedfor printing. The drops 33 result from intermittent breaking up of thejet creating an isolated drop, called an intermittent drop 33. Theelectric charge of the intermittent drops 33 is quasi-zero as they areformed in the first breaking position of the jet, facing the conductor34 brought to the same potential as the ink found in the chamber 28,generally the zero potential of the electrical ground. Afterintermittent break-up, the jet 30 is split into the drop 33 and a jetsection 38.

The drops 43 are those which are not used for printing. They are formedat the second breaking position, facing the conductor 37 of the chargingelectrode 35 brought to the non-zero electrical potential Vc, differentfrom the one of the ink found in chamber 28. The drops 43 are loaded byelectrostatic influence with a larger electric charge in absolute valuethan the quasi-zero charge loaded by the drops 33. The second breakingposition 12 where the drops 43 are formed, is downstream from the firstbreaking position 11 where the intermittent drops 33 are formed. Thisbreak-up is called a continuous downstream break-up of the jet sections38, or of the jet 30 if the intermittent break-up has not formed anysections. All the drops which separate from the jet then pass into thedeflection area defined by the deflection electrode 39. The ink drops33, 43 passing through the deflection area are subject to anelectrostatic force F=q.E0, q being the electrical charge of therelevant drop. The intermittent drops 33, the electrical charge of whichis quasi-zero, therefore follow a quasi-rectilinear trajectory along theaxis of the nozzle 29, up to the printing medium 41. The trajectories ofthe drops 43 are themselves deflected perpendicularly to the axis of thejet according to their electrical charge and end their trajectory in therecovery trough 40, assuming that a suitable combination of electricalpotentials is applied to the charging and deflection electrodes 35, 39.The ink collected in the trough 40 is, in a known way, re-injected intothe ink circuit in order to be reused.

The printing of a pattern in a known way per se results from theselection of ink drops to be directed towards the printing medium 41 ortowards the trough 40 and from a relative movement of the printingmedium 41 and the printing head 1. In the example commented above, theuncharged drops, the trajectory of which is not deflected, are the oneswhich are used for printing. Generally, this solution is preferred asthe accuracy of the positioning of the drops contributing to theprinting is higher, because the trajectory of these drops is shorter andless dependent on uncertainties relative to the exact mass of the drop,to the value of the drop-borne amount of electric charge and possiblefluctuations of the deflection field. According to the invention, theuse, as in certain known embodiments, of deviated drops for printing isnot excluded while the undeviated drops are directed towards the trough.

One of the main advantages of the invention is that, as in the secondinvention of Vago, the set of charging 35 and deflection 39 electrodes,forming together a system for sorting drops 33 for printing and drops 43to be recovered, is common to all the jets. However, because thesections 38 formed whenever an intermittent drop 33 is formed, are, in adownstream position, also fractionated into drops 43, the trough 40common to all the jets may be of a more reduced size as the drop guidingaccuracy is enhanced.

FIG. 2 is intended to illustrate the breaking modes of the jet in orderto form the intermittent 33 and continuous 43 drops. In FIG. 2 portiona, a phase is found in which there is no printing, or in which there hasnot been any intermittent break-up during the time taken by the jet formoving from the upstream breaking position 11 to the downstream breakingposition 12. In this case alone, a periodic signal continuously breaksthe jet at the downstream position 12 in order to form continuous drops43. In FIG. 2, portion b, the case is illustrated when a drop 33 is forexample formed by a pulse of the breaking signal. In this case, the jet30 is split into a drop 33 and a section 38 of the jet. This sectionbears the velocity perturbation provided by the periodic signal. It istherefore broken up at the downstream breaking position 12 in order toprovide continuous drops 43. Thus, downstream from the downstreambreaking position, the jet is entirely divided into drops 33 and 43.

Forms of electrical signals able to cause the intermittent breaking atthe upstream breaking position 11 on the one hand, the downstreamcontinuous breaking at the downstream breaking position 12 on the otherhand, and finally a composition of the breaking signals in the upstreamand downstream positions, will now be discussed.

First it should be noted that the intermittent break-up is a break-upintended to isolate a drop from a jet. This situation is different fromthe situation where a continuous train of drops is generated, because,in the case of the isolated drop, satellite droplets and beads tend toform, which are detrimental to the printing quality. To understand thebenefit of possible forms of the intermittent breaking signal, thebreaking dynamics of an isolated drop will be described hereafter inconnection with FIG. 3, which for the invention corresponds to the caseof the intermittent drop.

FIG. 3 includes portions a to g. The sequence of portions a to g show atime succession of states of the intermittent break-up for presentingthe dynamics of break-up. In a first stage illustrated in a, a velocityperturbation provided by an induced temporary overpressure, at thechamber 28, generates a ventral segment 33 a in the jet.

An intermittent drop 33 consecutively separates at two breaks: anupstream break 49 illustrated in portion b by a space between theupstream portion of the jet 30 and the downstream portion, and adownstream break 50 illustrated in portion c by a space between the drop33 which is formed at this stage and the downstream portion of the jet30 which therefore becomes a jet section 38. Upstream 51 and downstream52 ligaments illustrated in portions b and c which respectivelycorrespond to stretches of upstream and downstream portions of the jet30 relatively to the forming drop 33, may, if stretching is significant,respectively give rise to upstream 53 and downstream 54 satellitedroplets illustrated in portion d. In portion d, it is also seen thatthe upstream and downstream portions of the jet on either side of theforming drop 33 are subject to swelling. As illustrated by thesuccession of states illustrated in portions e and f, these swellings ofthe ends of the jet and of the jet section surrounding the forming drop33, may also separate in order to form ink drops 55, 56 illustrated inportions g. These upstream and downstream ink drops 55, 56 willsubsequently be called upstream bead 55 and downstream bead 56. Anupstream breaking length Lbam is defined as being the Lbam distancebetween the output face of the nozzle 29 and the upstream break 49, adownstream breaking length Lbav is defined as being the Lbav distancebetween the output face of the nozzle 29 and the downstream break 50.

In order that the beads 55, 56 be recovered in the trough 40, it isnecessary that the latter bear a sufficient electrical charge, andtherefore that they separate sufficiently far downstream from theupstream and downstream breaks 49, 50 of the intermittent drop 33 sothat, at the moment of their separation from the jet, they are found inthe area where a potential different from the potential of the ink inthe chamber 29 exists. This is why in FIG. 3, portions f and g, thebeginning and the end of the separation of the beads 55, 56 areillustrated in the area subject to the influence of the electrode 37.Also, it is desirable that the upstream and downstream satellites 53, 54be rapidly absorbed in other drops, as they may significantly dirty thesorting system or even the printing medium.

Any electrical signal applied to the stimulation device 31 and withwhich break-up features may be obtained so that the satellites and beadsdo not introduce any printing defects as explained above, may be usedfor achieving the invention.

FIG. 4 portion a, shows an example of an electrical control signal whichmay be applied to the stimulation device 31 in order to control theshape of the intermittent breaks so as to ensure proper operation of thesorting of the drops to be printed 33 and of the drops 43 to berecovered in the trough 40.

The signal illustrated in FIG. 4 portion a, consists of threeconsecutive voltage steps with the respective levels U₁, U₂, and U₃,measured above a level U₀. The three steps have respective durations T₁,T₂, and T₃. Two consecutive steps are connected to each other by a steeprising or falling edge.

The durations T₁, T₂, and T₃ of three consecutive voltage steps whichform the stimulation signal are each close to a duration τopt. τopt isthe duration of a rectangular pulse which would give, if it was appliedto the stimulation means 31, the shortest upstream intermittent breakinglength, with a constant amplitude and for the same jet (same velocity,same section, same ink). τopt is a duration which corresponds to aspatial perturbation of the jet with a length of λopt/2, where λopt isthe optimum wavelength of the jet, i.e., the wavelength for which thecoefficient for amplifying the capillary instability is maximum.

As λopt≅10.a for a viscous liquid, one obtains τopt=λopt/2.Vj≅5.a/Vj.

As a reminder, in the above formulae, a is the equivalent diameter ofthe nozzle 29 which substantially corresponds to the diameter of the jet30 and Vj is the ejection velocity of the jet 30.

In the commented example in connection with FIG. 4 a, the characteristicdurations T₁, T₂, and T₃ were selected to be equal to each other, i.e.,T₁, T₂, and T₃=τopt, so the shape of the obtained break for forming anintermittent drop 33 is stable, and therefore not very sensitive toslight variations of the jet velocity, of the viscosity or otherfluctuating properties of the jet.

In addition, the principle of sorting the drops requires that theelectrical charge borne on the intermittent drop 33 is quasi-zero inthis example. Now, the electric charge actually borne on this dropdepends on the geometrical configuration of the charging electrode 35,on the electrical potentials applied to the 2 conductors 34, 37 whichform it, but also on the algebraic distance between the upstream anddownstream intermittent breaks (Lbav−Lbam).

With the signal illustrated in FIG. 4 portion a, this distance(Lbav−Lbam) between the two breaks forming an intermittent drop may becontrolled so as to ensure a stable and well defined trajectory of thedrop to be printed.

The distance (Lbav−Lbam) between the upstream and downstream breaksforming a drop may be adjusted by changing certain parameters of thestimulation signal. In this embodiment, adjustment of the amplitude U₁,U₂, and U₃ of the steps forming the pulse signal allows adjustment of(Lbav−Lbam). More specifically, a reduction of the absolute value of theabsolute difference |U1−U2| between the voltage values of the first twosteps results in delaying the moment of the downstream break, and so areduction of the absolute difference |U2−U3| between the voltage valuesof the last two steps results in delaying the moment of the upstreambreak. It is possible to select T1=0 or T3=0, if one of the three stepsof the signal is estimated to be unnecessary by the skilledpractitioner, depending on the particular operation of the relevantstimulation device. With the signal shown, it is possible to correct thetrajectory of the drop to be printed by empirically selecting theparameters of the signals which have an influence on the distance(Lbav−Lbam) between the upstream intermittent break and the downstreamintermittent break.

Another example of a pulse stimulation signal which may be used in oneembodiment of the invention is described in FIG. 4 portion b. Thissignal consists of a succession of three rectangular pulses, a firstpulse with duration D₁ and level U₁, a second one with duration T₂ andlevel U₂, and a third one with duration D₂ and level U₃. The first andsecond pulses are separated from each other by duration Tr₁, and thesecond and third pulses are separated from each other by duration Tr₂.During the separation times between pulses, the signal is at the baselevel U₀. If this signal is selected for controlling the intermittentbreak, the durations preferably are T₂≅τopt; Tr₁≅Tr₂≅τopt/2; D₁ and D₂are close to τopt/10 or τopt/5 according to the stimulation device to becontrolled, τopt being defined as earlier. The distance between theupstream and downstream breaks of the intermittent drop 33 may then beadjusted by changing U₁ and/or U₃: the instant of the downstream breakis delayed when U₁/U₂ increases, the instant of the upstream break isdelayed when U₃/U₂ increases.

We will now proceed with describing a signal able to generate thebreaking of the jet or jet sections in the second so-called downstreamposition, producing drops 43 which will be recovered by the trough 40.

Application of a simple sinusoidal signal would cause generation ofsatellite droplets between the main drops 43 issued from this break. Inthe embodiment described here, continuous break-up without any satellitewith a sufficiently weak amplitude signal in order to place thedownstream continuous break-up in the vicinity of the charging conductor37 is obtained by applying a signal with two modes, superimposition oftwo sinusoidal signals with frequencies Fb and 2.Fb, with properlyselected relative amplitudes and phase lags. The generated signal hasthe form:Sb(t)=Ab.(sin(2π.Fb.t)+α.sin(4π.Fb.t+φ)  (1)

In formula (1) above, Fb=1/Tb is the fundamental frequency of thecontinuous stimulation signal for forming drops 43. α>0 is the relativeamplitude of the second mode, and φ is its relative phase. Ab is acoefficient which determines the amplitude of the continuous stimulationsignal for forming drops 43. The skilled practitioner knows how toselect the values of parameters α and φ in order to obtain a continuousbreak-up without any satellite droplets. A signal such as describedabove, is illustrated in FIG. 5 portion a. This is a periodic signalwith period Tb, the amplitude of which versus time is illustrated byformula (1). If this signal is applied alone continuously, breaking ofthe jet is obtained as illustrated in FIG. 2 portion a, where only drops43 are produced.

The combination of the signals for generating drops 33 and 43 will nowbe explained. The time combination of both time signals, theircombination from the point of view of relative amplitudes and finally acontrol mode for introducing a pulse signal into a succession ofperiodic signals will be successively examined.

From the time point of view, at least one period Tb of the downstreamcontinuous stimulation periodic signal is for obtaining an intermittentdrop replaced, for example, by the pulse control signal described inconnection with FIG. 4 portion a. The combination of the pulse signaldescribed in connection with FIG. 4 portion a, and of the periodicsignal described in connection with FIG. 5 portion a, is illustrated inFIG. 5 portion c. As shown by the example of FIG. 5 portion c, the totalduration of the intermittent stimulation signal is equal to a value Ti.It is formed as illustrated in FIG. 4 portion a, by a succession ofthree consecutive steps with respective durations T₁, T₂, and T₃, T₃having in this example zero duration, so that Ti=T₁+T₂+T₃. As a rule,Ti≠n.Tb, n being an integer. In the selected embodiment, the pulsestimulation signal is preceded by a downstream transition signal withduration tav, and followed by an upstream transition signal withduration tam. Durations tav and tam are selected so as to satisfy thecondition tav+Ti+tam=n.Tb. In the example described in connection withFIG. 5 portion c, the transition signals simply consist of holding thevoltage constant between the interruption of the continuous stimulationperiodic signal and the beginning of the generation of the pulse signal.Durations tav and tam are selected so as to observe the integrity of thejet sections 38 on either side of the intermittent drop 33 up to thearea of influence of the charging conductor 37 (FIG. 1). The transitionsignals are also selected so as to ensure continuity of the appliedelectrical signal to the stimulation means 31 during interrupting andresuming generation of the downstream continuous stimulation periodicsignal. It is noted that the transition signals may either one of themor both, have zero duration.

The relative amplitudes of the periodic signal and of the pulse signal,i.e., the relative values of Ab in formula (1) defining the periodicsignal and the value of U2 are selected in order to properly place theupstream and downstream break positions in the areas of influence of thecharging electrode 35. The breaking lengths, i.e. the distance betweenthe nozzle 29 and a breaking position, depend on the amplitude of thestimulation. To ensure effective separation of the drops 33 relativelyto the drops 43, the distance between the intermittent breaking position11 and the downstream continuous breaking position 12 should besufficient, at least 20 times the radius of the jet. In the preferredembodiment, a distance between these two breaking positions is close to50 times the radius of the jet.

The electrical control signal generator 32 able to generate on demandthe pulse signal for generating an intermittent drop 33 and the periodicsignal for continuous generation of drops 33 and connected for thispurpose to the stimulation means 31, is, in the described embodiment,driven by means of a printing command, for example a logic signal, forexample an IMP binary signal illustrated in FIGS. 5 b and 5 d. SignalIMP is a function of the data to be printed. When only the downstreamcontinuous stimulation signal with period Tb is generated, the logicalvalue of the Boolean signal IMP remains 0. This is the constantly 0signal which is illustrated in FIG. 5 b.

In the printing situation, the IMP signal switches to the value 1 duringat least one period Tb, triggering the response of the electricalcontrol signal generator 32: thus, according to the preferred embodimentof the invention, the generator 32 of signals for controlling thestimulation means 31 is able to combine a signal with a pulse nature anda periodic signal, by replacing an integral number n of periods of theperiodic signal with the pulse signal flanked with transition signals.

Enhancements which may be made to the printing head according to theinvention will now be examined in connection with FIGS. 6 and 7, whichrespectively illustrate an example of a section of a printing head 1showing the path of the ink in a body 23 of this head and a graphshowing in abscissae the velocity perturbation locally brought to thejet depending on the frequency of a mechanical stimulation present inthe ink circuit upstream from the nozzle.

The hydraulic path inside the body 23 of the printing head 1 illustratedas a sectional view in FIG. 6 along one or more xz planes, z being thedirection of the jet 30 and x a direction perpendicular to z located ina plane perpendicular to the plane containing the axes of the nozzles29, includes from upstream to downstream discrete functional componentsin the direction of flow of the ink. A reservoir 17 of pressurized ink16 is in communication, as illustrated by arrows 27, with an ink feedingconduit not shown. The reservoir 17 is in communication with a narrowpassage 18 called a restrictor. A first connecting tube 20 puts therestrictor 18 into communication with the stimulation chamber 28. Thestimulation chamber 28 is itself in communication with the nozzle 29 forforming the jet 30, via a second connecting tube 21. The nozzle 29 ispierced in a nozzle plate 22 which may include several nozzles alignedalong a direction y perpendicular to the representation plane xz.

A wall portion of the chamber 28 is formed by a membrane 24, thethickness of which, along the Z axis, is much smaller than itsdimensions in the X,Y planes. A piezoelectric component 25 is stuck onthe external face of the membrane 24, i.e., the one which is external tothe chamber 28.

When an electrical signal is applied on the piezoelectric component 25,the pair membrane 24/piezoelectric component 25 which forms in thisexample the stimulation means 31, forms a vibrating component 31 whichdeforms in flexion with the effect of producing a modulation of thevolume and pressure within the chamber 28; this results in a modulationof the average ejection velocity of the ink 16 at the nozzle 29. Thistype of actuator which is described in many patents was initiallyproposed by Silonics (U.S. Pat. No. 3,946,398—Kyser & Sears).

The requirement of forming an isolated drop in a jet by applying anintermittent signal, as described in FIG. 4 portion a or b, andpreferably avoiding the formation of satellite droplets such as 53, 54described in connection with FIG. 3, as well as the formation of a trainof drops behind the isolated drop, requires that the stimulation bestrictly non-resonant. This means that the transfer function of thestimulation system should be free of any resonance peaks in thebandwidth of the jet 30. The transfer function of the stimulation systemis defined as the relationship existing between the pressure induced bythe action of the piezoelectric component 25 and the modulation of theejection velocity of the jet 30.

The definition of the bandwidth BP_(jet) of the jet 30 is derived fromthe linear theory of capillary instability, the skilled practitionerwill know how to recall the following relationship: $\begin{matrix}{{BP}_{jet} \in \left\lbrack {0;{Fc}_{jet}} \right\rbrack} & \quad & \quad & {{Fc}_{jet} = \frac{V_{jet}}{2\quad\pi\quad R_{jet}}}\end{matrix}$

For the numerical application:

Vjet: velocity of jet 30, for example 15 m/s

Rjet: radius of the jet at the nozzle output 29, for example 15 μm.

Fcjet=cut-off frequency of the jet, for example 160 kHz.

The stimulation system is capable of producing resonance frequencies FRrelated to the mechanical and acoustic behavior of the device. In orderto obtain a strictly non-resonant stimulation, one will seek to placethese resonance frequencies FR outside the bandwidth of the jet.Preferentially, the following relationship will be satisfied:F _(R)>(1+0.1) Fc _(jet)

For this, one will strive to comply with one or several of the designrules hereafter.

Resonance of Mechanical and Acoustic Origin (Design Rule No. 1)

The vibrating component has a resonance eigenfrequency F_(M) whichmainly depends on its geometry and on the mechanical properties of thematerials which compose it.$F_{M} = \frac{1}{2\quad\pi\sqrt{\left. {L_{M}*C_{M}} \right)}}$

L_(M): an inertial term equivalent to an inductor in an electricanalogy.

C_(M): an elasticity term equivalent to a capacitor in an electricanalogy.

With the nominal values indicated in a dimension and material tablesubject of annex 1, the resonance frequency of the vibrating component31 is typically of the order of 400 kHz.

In the absence of any propagation phenomenon, the Helmholtz frequencyF_(H) calculated from the inertial and elasticity terms (electricanalogy) of each discrete component forming the simulation device, i.e.,the restrictor, the chamber and the nozzle as well as the hydraulicconnecting components between these components if they exist, will be ofinterest.

With the nominal values indicated in the dimension and material table,the Helmholtz resonance frequency which is typically of the order of 200kHz is located outside the bandwidth of the jet. In the particular caseof the values proposed in the table subject of annex 1, the Helmholtzfrequency F_(H) is calculated from the following simplified expressionwhich only retains terms with preponderant weights:$F_{H} = \frac{1}{2\quad\pi\quad\sqrt{\left( \frac{L_{B}L_{R}}{L_{B} + L_{R}} \right)*C_{M}}}$

L_(R): inertial term (electric analogy) associated with the restrictor18.

L_(B): inertial term (electric analogy) associated with the nozzle 29.

C_(M): elasticity term in the electric analogy of the vibratingcomponent 31.

Acoustic Resonance with Propagation (Design Rule No. 2)

Acoustic propagation phenomena may produce resonance peaks when one ofthe characteristic wavelengths of the stimulation system is notinsignificant relatively to the wavelength λ of the acoustic waves inthe ink 16. As an example, the wavelength λ is typically 7.5 mm in anink based on water, MEK or alcohol for a 160 kHz cut-off frequency ofthe jet Fc_(jet) and for an average sound velocity, for example in MEK,of 1,200 m/s. A characteristic length means any dimension of therestrictor 18, of the chamber 28, of the first and second connectingtubes 20, 21, of the nozzle 29 and of the total path of the ink 16 inthe stimulation system from the inlet of the restrictor 18 to the outletof the nozzle 29. Ideally, all the characteristic lengths of thestimulation system will be less than λ/4 in order to be rid of acousticwave propagation. The λ/4 constraint sets the maximum characteristiclength to 1.8 mm. Generally, it is easy to satisfy the λ/4 constraintfor the nozzle 29, the restrictor 18 and the connecting tubes 20, 21, asindicated in the appended dimension and material table. For the chamber28, this rule may not be observed, as a large surface of the chamber issought after in order to obtain proper stimulation efficiency, in thiscase, it is absolutely necessary to proceed with modeling of thetransfer function in order to ensure that there is no resonance in thebandwidth of the jet.

For a stimulation system including the nominal dimensions indicated inthe dimension and material table, it appears that its transfer function,the curve of which is shown in FIG. 7, does not have any resonantfrequency in the bandwidth of the jet, a resonance peak 26 at 200 kHzassociated with the Helmholtz frequency.

For a 160 kHz cut-off frequency of the jet and for a stimulation systemwith the indicated dimensions in the table of annex 1, the firstresonance is located around 200 kHz which meets the listed criteria andprecautions, it is easily to check that the stimulation is not resonantand so it is possible to advantageously form a drop in a continuous jet(FIGS. 6 and 7).

Optimization of the Stationary and Unstationary Flow (Design Rule No.4).

Under the effect of the piezoelectric component 25, a pressure pulsepushes ink 16 towards the nozzle 29 and pushes ink 16 back towards therestrictor 18, indeed both of these two components form, for the chamber28, the two output points of the ink 16. In order to maximize theefficiency of the stimulation, i.e., the velocity modulation at nozzle29, it is desirable to match the impedance of the nozzle 29 to that ofthe restrictor 18 which has high acoustic impedance. The yield of thestimulation will be defined by the ratio R_(imp) of the impedances L_(B)of the nozzle and L_(R) of the restrictor 18:$R_{imp} = {\frac{L_{R}}{L_{B}} = {\frac{l_{R}}{S_{R}}\frac{S_{B}}{l_{B}}}}$

In the above formula:

l_(R): length of the restrictor 18

l_(B): length of the nozzle 29 in the Z direction

S_(R): cross section of the restrictor 18

S_(B): cross section of the nozzle 29

With the idea of maximizing R_(imp), the intuitive solution which wouldconsist of selecting l_(R)>>I_(B) and S_(R)<<S_(B) is of no interest asit requires a too large pressure of ink in the reservoir 17. Indeed, theformation of the continuous jet 30 requires a static ink pressureupstream from the restrictor 18 which strongly depends on the viscouspressure drops in the stimulation system and in particular in the nozzle29 and the restrictor 18 which are the two areas with the higher flowvelocity of the ink. The hydraulic resistance of the nozzle 29 or of therestrictor 18 is described, in a first approximation, by Poiseuille'slaw according to the following generic expression:$\frac{\Delta\quad P}{Q} = {R_{Hydro} = \frac{8\quad\mu\quad l}{\pi\quad R^{4}}}$

ΔP: static pressure drop between the inlet and outlet of the nozzle 29or of the restrictor 18

Q: volume flow rate

R: radius of the nozzle 29 or of the restrictor 18

l: length of the nozzle 29 or of the restrictor 18,

μ: is the dynamic viscosity of the ink.

In order to reduce the hydraulic resistance of the restrictor 18comparatively to the nozzle 29 and while retaining a good stimulationyield, one will act on the equivalence [length←→section] by preferring asection and a length of the restrictor 18 larger than that of the nozzle29. The nominal dimensions indicated in the dimension and material tableof annex 1, are a good compromise between the stimulation yield and theviscous pressure drop. For nozzle 29 and restrictor 18 radius,respectively length, ratios of typically 1/3, respectively 1/10, oneobtains: $\left\{ \begin{matrix}{R_{Imp} \cong 1} \\{{R_{Hydro}({restrictor})} \cong {\frac{1}{10}{R_{{Hydro}\quad}({nozzle})}}}\end{matrix}\quad \right.$

The volume contained in the chamber 28 with a parallelepipedous shape isselected so that the Helmholtz frequency of the system is not less than200 kHz. The thickness of the chamber (in the Z direction) should be assmall as possible in order to provide a maximum surface to the vibratingcomponent 31 but nevertheless not less than the diameter of the nozzle29 in order to minimize the viscous pressure drop in the chamber 28.This thickness which results from a compromise, will be selected so asto be close to the diameter of the nozzle 29. As the volume andthickness are given, this sets the surface of the chamber while ensuringgood consistence with design rule No. 1.

Thus, a printer according to the invention includes:

a device for ejecting liquid with which at least one ink jet may beformed,

a generator of electrical control signals,

an internal stimulation device, i.e., upstream from the nozzle, withwhich the jet may be fractionated by creating perturbations at itssurface at the output of the nozzle. This stimulation device is capableof generating an isolated drop in the jet when the suitable pulse signalis applied on the stimulation means,

a sorting system consisting of an arrangement of electrodes brought toconstant electric potentials and of a trough which collects theunprinted drops.

With the invention, it is possible to use a common sorting system for alarge number of jets, which eliminates the difficulties in themanufacture of charging electrodes of a conventional binary printer, andto make the most of the advantageous of the sorting system withintermittent stimulation, notably its low manufacturing cost. Further,as the stimulation is internal, the problems of bulkiness anddifficulties related to external stimulation techniques are eliminated.With the stimulation device driven according to the principle of theinvention, it is also possible to change the behaviour of the jet andtrajectory of the drops by the sole means of the stimulation signal,which simplifies the electronic portion of the printing head andprovides very fine control over the stability of the jet and theprinting quality. The combination of two stable breaks also contributesto controlling the two trajectories of the two types of drops created bysimple adjustment of the stimulation signal parameters, which contributeto enhancing the reliability of the machine and the printing quality.

It will be noted that a printing head using the invention may eithercomprise or not the circuit 32 for generating break-up signals.

Annex 1

Dimension and material table Length (X)/ Thickness Function Width(Y)/radius (Z) Materials Restrictor 18 250 μm/130 μm/— 38 μm Inox 316Connecting tube 20 —/—/75 μm 38 μm Inox 316 Chamber 29 1000 μm/410 μm/—38 μm Inox 316 Vibrating 1000 μm/410 μm/— 125 μm PZT component 31: -1000 μm/410 μm/— 62.5 μm Inox 316 piezoceramic- membrane Connecting tube21 —/—/50 μm 475 μm Inox 316 Nozzle 29 —/—/15 μm 50 μm Inox 316Inox: Stainless steel

Annex 2

List of cited documents US-A-4,220,958 CROWLEY US-A-3,596,275 SWEETUS-A-4 638,328 BRAKE ET AL. FR 2 799 688 → 09/685,064 Journal XeroxDisclosure (Pincus - 1982, vol. 7, p. 23).

1. An ink jet printer (10) comprising: a printing head (1) with one ormore nozzles (29) having a head (1) body (23) notably accommodating foreach nozzle 29), a hydraulic path of the ink including, a stimulationchamber (28) in hydraulic communication with one of the printing nozzles(29) emitting a pressurized ink jet (30) along an axis of this nozzle(29), internal means (31) for stimulating the ink jet (30) emitted bythe nozzle (29), mechanically coupled with the ink (16) accommodated inthe stimulation chamber (28), these means (31) acting on the jet (30)emitted by the nozzle (29) in order to break up the jet (30) in acontrolled way, and means (40) for recovering the ink which is notreceived by a printing substrate (41), a generator (32) of electricalcontrol signals receiving a control signal and delivering to stimulationmeans (31), stimulation signals, an arrangement (35) of chargingelectrodes defining around the axis of the nozzle (29), upstream anddownstream areas, the downstream area being further away from the nozzlethan the upstream area, upstream and downstream electrodes (34, 37) ofthis arrangement (35) being connected to sources of electric potentialin order to maintain in one of the areas, a potential equal to that ofthe ink found in the body (23) of the printing head (1), and in theother one of the areas a potential different from that of the ink foundin the body (23) of the printing head (1), an arrangement (39) ofdeflection electrodes axially located downstream from the arrangement(35) of charging electrodes, characterized in that the generator (32) ofelectrical control signals delivers to the stimulation means (31),signals intermittently causing controlled breaking of the jet (30) in anupstream breaking position (11) located in the upstream area, tointermittently form a drop, thereby separating the jet into a drop and asection, and also causing controlled breaking of the jet (30) or ofsections (38) of the jet (30) continuously in a downstream breakingposition (12), the continuous jet (30) emitted by the nozzle (29) beingthereby transformed after the downstream area into a continuous train ofelectrically charged and uncharged ink drops (33, 43).
 2. The printer(10) according to claim 1, characterized in that the upstream electrode(34) of the arrangement of charging electrodes (35) is connected to thesame potential as the ink (16).
 3. The printer (10) according to any ofclaims 1 or 2, characterized in that the stimulation means (31) includea piezoelectric material (25), the generator (32) of electrical controlsignals delivering to the stimulation means (31), a continuous printingsignal formed by a periodic signal of period T_(b), intermittentlyreplaced with a pulse signal preceded and followed by transitionsignals.
 4. The printer (10) according to claim 3, characterized in thatthe pulse signal delivered by the generator (32) of electrical controlsignals is formed by a pulse including three consecutive voltage stepsconnected from one to the next by a steep rising or falling voltageedge.
 5. The printer (10) according to claim 3, characterized in thatthe pulse signal delivered by the generator (32) of electrical controlsignals is formed by a succession of three rectangular pulses separatedfrom each other by voltage steps with a level than the level of thepulse with the lowest level.
 6. The printer (10) according to any ofclaims 3 to 5, characterized in that the periodic signal delivered bythe generator (32) of electrical control signals is formed by acombination of two sinusoidal signals.
 7. The printer (10) according toany of claims 3 to 5, characterized in that the periodic signaldelivered by the generator (32) of electrical control signal is formedby a combination of more than two sinusoidal signals.
 8. The printer(10) according to any of claims 3 to 5, characterized in that the sum ofthe durations of the pulse signal and of the transition signalsdelivered by the generator (32) of electrical control signals is equalto an integral number of periods of the periodic signal.
 9. The printer(10) according to any of claims 1 to 8, characterized in that aHelmholtz frequency of a portion of a hydraulic path of the ink feedinga nozzle (29) comprising a restrictor (18) and the portion locateddownstream from this restrictor (18) has a value located outside abandwidth of the jet (30) issued from this nozzle (29).
 10. The printer(10) according to any of claims 1 to 8, characterized in that thehydraulic path of the ink includes a restrictor (18) and in that thelength of a hydraulic path between an inlet of the restrictor and thenozzle (29) is less than the quarter of the wavelength of sound in theink.
 11. The printer (10) according to any of claims 1 to 8,characterized in that the system for stimulating a jet (30) emitted by anozzle (29) is strictly non-resonant.
 12. The printer (10) according toany of claims 3 to 8, characterized in that the stimulation means (31)include, in addition to the piezoelectric material (25), a membrane (24)which is mechanically coupled with it, a resonance frequency of avibrating component formed by the membrane (24) and the piezoelectricmaterial (25) is larger than a cut-off frequency of the jet (30).
 13. Amethod for printing a medium by means of a printer (10) according to anyof claims 1 to 11, wherein an ink jet (30) emitted by a nozzle (29) ofthe printer is fractionated in order to form first drops (33), impingingon a printing substrate in order to form points (58) and sections (38),characterized in that, the jet (30) or the sections (38) resulting fromthe fractionation of the jet into first drops (33) and of sections (38)are further fractionated, the second drops (43) resulting from this lastfractionation, being directed towards the trough (40).
 14. An ink jetprinter (10) head (1) comprising: a printing head (1) with one or morenozzles (29) and a head (1) body (23) notably accommodating for eachnozzle (29), a hydraulic path of the ink including a stimulation chamber(28) in hydraulic communication with one of the printing nozzles (29)emitting a pressurized ink jet (30) along an axis of this nozzle (29),internal means (31) for stimulating the ink jet (30) emitted by thenozzle (29) mechanically coupled with the ink (16) accommodated in thestimulation chamber (28), these means (31) acting on the jet (30)emitted by the nozzle (29) for breaking up the jet (30) in a controlledway, and means (40) for recovering the ink which is not received by aprinting substrate (41), a generator (32) of electrical control signalsreceiving a control signal and delivering stimulation signals to thestimulation means (31), an arrangement (35) of charging electrodesdefining around the axis of the nozzle (29), upstream and downstreamareas, the downstream area being further away from the nozzle than theupstream area, upstream and downstream electrodes (34, 37) of thisarrangement (35) being connected to sources of electric potential so asto maintain in one of the areas a potential equal to that of the inkfound in the body (23) of the printing head (1), and in the other one ofthese areas, a potential different from that of the ink found in thebody (23) of the printing head (1), an arrangement (39) of deflectionelectrodes axially located downstream from the arrangement (35) ofcharging electrodes characterized in that the generator (32) ofelectrical control signals delivers to the stimulation means (31),signals intermittently causing controlled breaking up of the jet (30) inan upstream breaking position (11) located in the upstream area, andalso causing controlled breaking up of the jet (30) or of the sections(38) of the jet (30) continuously in a downstream breaking position(12), the continuous jet (30) emitted by the nozzle (29) being alsotransformed after the downstream area into a continuous train ofelectrically charged and uncharged ink drops (33, 43).
 15. The printer(10) head (1) according to claim 14, characterized in that the upstreamelectrode (34) of the arrangement of charging electrodes (35) isconnected to the same potential as the ink (16).
 16. The printer (10)head (1) according to any of claims 14 or 15, characterized in that thestimulation means (31) include a piezoelectric material (25), thegenerator (32) of electrical control signals delivering to thestimulation means (31), a continuous printing signal formed by aperiodic signal with period T_(b), intermittently replaced with a pulsesignal preceded and followed by transition signals.
 17. The printer (10)head (1) according to claim 16, characterized in that the pulse signaldelivered by the generator (32) of electrical control signals is formedby a pulse including three consecutive voltage steps connected from oneto the next by a steep rising or falling voltage edge.
 18. The printer(10) head (1) according to claim 16, characterized in that the pulsesignal delivered by the generator (32) of electrical control signals isformed by a succession of three rectangular pulses separated from eachother by voltage steps with a level less than the level of the pulsewith the lowest level.
 19. The printer (10) head (1) according to any ofclaims 16 to 18, characterized in that the periodic signal delivered bygenerator (32) of electrical control signals is formed by a combinationof two sinusoidal signals.
 20. The printer (10) head (1) according toany of claims 16 to 18, characterized in that the periodic signaldelivered by the generator (32) of electrical control signals, is formedby a combination of more than two sinusoidal signals.
 21. The printer(10) head (1) according to any of claims 16 to 18, characterized in thatthe sum of the durations of the pulse signal and the transition signalsdelivered by the generator (32) of electrical control signals is equalto an integral number of periods of the periodic signal.
 22. The printer(10) head (1) according to any of claims 14 to 21, characterized in thatthe Helmholtz frequency of a portion of a hydraulic path of the inkfeeding a nozzle (29) comprising a restrictor (18) and the portionlocated downstream from this restrictor (18), has a value locatedoutside a bandwidth of the jet (30) issued from this nozzle (29). 23.The printer (10) head (1) according to any of claims 14 to 21,characterized in that the hydraulic path of the ink includes arestrictor (18) and in that the length of a hydraulic path between aninlet of the restrictor and the nozzle (29) is less than the quarter ofthe wavelength of sound in the ink.
 24. The printer (10) head (1)according to any of claims 14 to 21, characterized in that the systemfor stimulating a jet (30) emitted by a nozzle (29) is strictlynon-resonant.
 25. The printer (10) head (1) according to any of claims16 to 21, characterized in that the stimulation means (31) include inaddition to the piezoelectric material (25), a membrane (24) which ismechanically coupled with it, and in that a resonance frequency of avibrating component formed by the membrane (24) and by the piezoelectricmaterial (25), has a value located outside a bandwidth of the jet (30).